The same word describes the best and the worst of Islam. Jihad refers both to holy war, directed against infidels, and inner struggle directed against one's lesser nature: defiance of satanic temptation. Of course this is basic to other religions too, but Muslims seem to foreground this theme in a natural, confident way. It is an important contribution to British culture, which has very largely forgotten about its own traditional concept of spiritual struggle.

The need for jihad (in the inner sense) is signalled in a drumbeat of depressing headlines: anger is rising, depression is rising, obesity is exploding, alcoholism is gripping young people earlier, British kids swap STIs before they've finished swapping sweets. We are the least self-disciplined nation in Europe. And it's happened quite quickly. We used to be the most restrained. We used to feel that Brief Encounter said something about the national soul. It looks as if we have rebelled against our former image, like a convent girl gone slutty.

The problem is that we can't go back to our old model of self-restraint, the stiff upper-lip one, for the culture of openness that arose in the 60s can't be shut down. We can't revive the ideal of conformity to public decency or bourgeois order, because they've been disturbed by cultural change, for better or for worse (largely for better, it has to be said). We need a new idiom of self-restraint and moral striving.

Or rather we need to get back to the older idiom, the one based on the mythological battle of good and evil. And here is where the Muslim idea of spiritual jihad can show us the way, or remind us of it. We need to revive the idea that moral and psychological effort is normal, the basic plot of life. And I think this idea is reliant on the narrative, or myth, of the struggle of good against evil.

We ought to see depression in these terms. It is not an illness suffered by a minority, but something that every grown-up has to contend with from time to time. By pathologising it we imply that normal people are naturally happy, which adds to the problem: some young people feel incapable of this supposedly normal state of easy happiness, and despair. It would be healthier to teach young people that it's normal to suffer bouts of gloom, and that the proper response is to toughen oneself against them, to fight back. And for this it helps to have a model of the self as a battleground between good and evil. Here we ought to learn from one of the masters of modern psychology, Martin Luther. When he felt gloomy he blamed Satan, who hates happiness just as he hates goodness and truth. For Luther, this is the essence of the spiritual life: resisting Satan's assaults on one's peace of mind.

Our culture perpetuates the myth that normal people are happy and decent with no effort, that inner struggle is unnecessary, that we do not need to pray to be delivered from evil, that the thoughts of our hearts do not need cleansing. This is the supreme lie of secular culture.

Many aspects of Christian morality, such as justice, equality, respecting victims and questioning authority have translated pretty well into secular terms. But the concept of inner struggle is something that secular morality fails to retain. And of course consumerism is a factor - advertising has made a huge assault on old virtues of patience, resisting temptation, suspecting one's desire. Go on, be a devil, you're worth it.

Literature used to recycle religious psychology and keep the theme of inner struggle in the cultural mind. It figured highly in 20th-century poets such as Kipling, Eliot and Auden. But in recent decades it's largely children's literature that recycles the mythological drama of fighting one's lesser nature.

And secular psychology doesn't help much. People like Oliver James try to explain that the consumer lifestyle makes us miserable, but such analysis lacks bite, unless it goes beyond criticising "the system", and confronts the darkness of our hearts. Auden's poem Song of the Devil nails the whole tradition of secular psychology. It imagines the devil boasting about how easy his job is nowadays: "Since social psychology replaced theology/ The process goes twice as quick,/ If a conscience is tender and loth to surrender,/ I have only to whisper: "You're sick!'"

Our culture needs to rediscover the idea of resisting temptation. It needs to see this not as a dull puritanical matter but as the supreme drama of our lives: participating in the cosmic defeat of evil. If we cannot revive our traditional idiom of jihad, there's a good chance that Islam will fill the vacuum.